Plasma-spraying devices or plasmatrons are used for low-power thermal spraying of powdered materials, for example in connection with different kinds of surface-coating. Such devices generally comprise a cathode, an anode and a plasma channel formed therebetween. During use of the device an electric arc is generated in the plasma channel, between the anode and the cathode, and gas is then introduced in the plasma channel for forming a plasma. The plasma jet thus flows through the plasma channel from an inlet end adjacent the cathode to an outlet end adjacent the anode. At the same time, a powdered material is supplied to the plasma jet for spraying thereof.
Today, one of two alternatives is used for supplying the powder.
According to the first alternative, the powder is introduced in the outlet area of the plasma channel, adjacent the anode. One advantage of this alternative is that when the powder is supplied the plasma flow is fully developed and has certain determined properties (temperature, velocity, sectional area, energy, etc.). These properties are dependent, inter alia, on the geometry of the plasma channel, the plasma-generating gas used and the amount of energy supplied. A further advantage of supplying the powder at the anode is that the heating of the plasma flow is not affected by the properties of the powdered material.
In connection with this variant of powder supply the powder is usually supplied perpendicularly to the plasma flow. The path of the powder particles travelling out from the anode area towards the surface to be coated will thus depend largely on the size and weight of the particles. The larger and heavier particles enter the high-temperature zone of the plasma jet directly, whereas the lighter ones first reach the centre of the plasma jet only in relatively cold zones located relatively far away from the anode. This means that there is a risk of part of the powder particles not being sufficiently hot and, moreover, of them missing the target, i.e. the object to be coated, for example, with the powdered material.
This is disadvantageous in that a large part of the powdered material is wasted, resulting in poor material economy. In other words, the powder-sprayed coating is produced using only a small part of the powder supplied. This is particularly disturbing when expensive coating materials are being used. The problem can be solved to some extent by using more homogeneous powders. A disadvantage associated with such powders, however, is that they are difficult to manufacture and, thus, relatively expensive.
To avoid problems associated with a perpendicular powder supply at the outlet area of the plasma channel, attempts have been made to provide a supply pipe for horizontal powder supply, which pipe is arranged directly in the plasma jet. However, one disadvantage hereof is that problems arise in connection with the heating of the plasma flow and that the plasma flow properties are greatly interfered with.
A further disadvantage generally associated with the introduction of the powdered material in the anode area, at the outlet of the plasma channel, is that a large amount of energy is needed to maintain the high temperature and specific power (power per unit of volume) of the plasma flow, so as to obtain, in turn, a homogeneous coating. It is believed that this is due to the fact that the plasma flow at the outlet of the plasma-spraying device, where the powder is supplied, has a virtually parabolic temperature and velocity distribution. Thus, the temperature and velocity gradient and the thermal enthalpy of the plasma flow are inversely proportional to the diameter of the plasma jet. To increase the homogeneity of the spray coating it is therefore necessary to increase the diameter of the plasma jet, which requires a lot of energy.
U.S. Pat. Nos. 3,145,287 and 4,445,021 discloses plasma-spraying devices in which the powdered material is introduced in the anode area, at the outlet of the plasma channel.
According to a second known alternative, the powder is supplied at the inlet of the plasma channel, at the cathode. In this case, the powder is heated by the electric arc simultaneously with the plasma-generating gas. The cathode area is considered to be a cold zone, which allows the powder to be introduced in the centre of the plasma flow.
When supplying gas at the cathode area in a plasma channel where an electric arc is generated at a predetermined discharge current, a small part of the gas will flow into the central part of the channel where the temperature is high, while the remaining part of the gas will flow along the channel walls, forming a cold gas layer between the channel walls and the electric arc. By using this gas distribution only a small part of the powder supplied at the inlet will flow into the electric arc, while the large part of the powder will flow in the cold layer adjacent the channel walls. This results in the powder being unevenly heated and the process being difficult to control. Furthermore, the channel and the anode risk being clogged by the powder, which thus has a detrimental effect on the conditions required for a stable plasma flow.
Trying to increase the transfer of mass to the central part of the channel by increasing the gas and powder flow is not a practicable alternative. The reason is that if the gas and powder flow is increased, while the current is kept constant, the diameter of the electric arc will decrease, which aggravates the problem of powder material accumulating in the cold areas along the channel walls. At the same time, the time during which the powder particles that actually end up in the heating zone remain in this zone decreases, since their velocity increases. This further reduces the quality of the process. Therefore, the amount of material in the hot zone cannot be increased if the current remains constant. Increasing the current implies, in turn, disadvantages both for the design and handling of the plasma-spraying device.
U.S. Pat. Nos. 5,225,652, 5,332,885 and 5,406,046 disclose plasma-spraying devices in which the powder is supplied at the cathode.
When analysing plasma-spraying processes, it has been found that the properties of the coating formed mainly depend on the thermal condition and velocity of the powder during spraying. The term “thermal condition” here primarily means the thermal profile and state of aggregation of the material. In prior-art plasma-spraying devices it is difficult, as described above, to control the thermal condition and velocity of the powder.